two black holes in highly elliptical orbit, so they they nearly collide when they come together. at the center of mass is a cloud of iron atoms. When the black holes nearly collide tidal forces across iron nuclei rip them apart. These ripped apart atoms can then be re-harvested and used as fuel in a fusion engine. The energy loss from ripping the atoms apart is then recovered by the black holes by taking advantage of dark energy when they go apart, which increases the distance between them, increasing the gravitational potential energy.

It would harness dark energy. /s

We haven't detected dark matter or dark energy outside of their visible effects on the larger universe. Maybe we can't interact with it with baryonic matter. A dark energy engine made out of dark matter would be invisible.